How do I access AngularJS' global `angular` variable in Angular and TypeScript?

I'm working on a big monolithic AngularJS application and want to add an Angular (as in Angular 4) component. Unfortunately, the AngularJS project's build system is largely fixed and the only thing I can do is adjust the project's index.html and, for instance, add additional <script> tags to load my Angular code. My question now is how I access AngularJS' global angular variable in my Angular project's TypeScript files to downgrade my Angular component and make it accessible to AngularJS, without adding the whole AngularJS library as a dependency to my Angular project since it's already bundled in the AngularJS project's JS files that the index.html loads.

To explain the situation a bit more in detail: I've followed the upgrade guide to bootstrap the AngularJS app from Angular, like so:

// src/app/app.module.ts of the Angular project
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { UpgradeModule } from '@angular/upgrade/static';

import { AppComponent } from './app.component';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    UpgradeModule
  ],
  providers: [],
})
export class AppModule {
  constructor(private upgrade: UpgradeModule) { }
  ngDoBootstrap() {
    // Bootstrap AngularJS app
    this.upgrade.bootstrap(document.documentElement, ['my-angularjs-app']);
  }
}

The files src/main.ts and src/app/app.component.ts are unchanged compared to a default $ ng new my-app project. Now, I run $ ng build --prod to produce production-ready JavaScript files that I can include in the AngularJS project's index.html as mentioned above. (Specifically, the Angular files are loader after the AngularJS files.)

Now I would like to make AppComponent available to AngularJS. This section of the guide suggests that I add the following lines to the above app.module.ts:

import { downgradeComponent } from '@angular/upgrade/static';

angular.module('my-angularjs-app')
  .directive(
    'my-ng2-component',
    downgradeComponent({ component: AppComponent }) as angular.IDirectiveFactory
  );

However, the angular variable obviously is not available and the TypeScript compiler complains about that. How do I change that? I've tried the following things to no avail:

1. Add a line declare var angular: any or a line let angular = window.angular. This results in Cannot find namespace 'angular' when adding the downgrade code from above.
2. Add @types/angular as a dependency to my Angular project and add a line import * as angular from 'angular'. (This results in Module not found: Error: Can't resolve 'angular' when adding the downgrade code.)
3. Like 2) but add a line /// <reference path="../../node_modules/@types/angular/index.d.ts" /> to the top of the file
4. Like 2) but add a line /// <reference types="angular" /> to the top of the file.

3) and 4) result in the following error message:

'angular' refers to a UMD global, but the current file is a module. Consider adding an import instead.

The latter seems to be related to the following two issues on GitHub:

• Built-in support for UMD module definitions
• Consider allowing access to UMD globals from modules

Is there any way that actually works? The upgrade guide, unfortunately, is very vague when it comes to this.